At present, cascaded multi-level technology, as a kind of mature technology, is well developed in fields of motor drive, medium voltage SVG and the like. In recent years, as a photovoltaic module is an independent direct current (DC) source, the cascaded multi-level technology is also widely used day by day in photovoltaic field. A cascaded multi-level inverter system can track a maximum power point of each photovoltaic module, and has a relative advantage in efficiency as a multi-level topology. The topology structure of the cascaded multi-level inverter system is shown in FIG. 1. However, there are some difficulties in applications of the cascaded multi-level inverter system in the photovoltaic field. For example, when the photovoltaic module fails, or is shielded, or is mismatched seriously, power of the photovoltaic module is unbalanced in different degrees, thereby leading to a problem of a low modulation voltage of the whole system and a modulation voltage saturation of an inverter module, and further seriously impacting working stability of a photovoltaic system which may result in a shutdown of the system for security.
To suppress the foregoing situations of power imbalance, a method of reactive power compensation is applied commonly in the prior art. The method is to inject capacitive or inductive reactive power into an output current, and compensate a modulation voltage to realize a stable operation of the whole system.
The above method of reactive power compensation to suppress the power imbalance may suppress the power imbalance with different degrees, but capacitive or inductive reactive power may also be injected into the power grid in the method, which is not allowed by the power grid.